As the number of electronic devices continue to multiply in residential and office environments, the adverse effects of electro-magnetic interference (EMI) noise from one piece of equipment on other nearby equipment are becoming more problematic. As a result, restrictions on permissable EMI levels produced by electronic devices are becoming more stringent which in turn is producing significant demand for high-efficiency power converters with appropriately reduced EMI emissions.
Forced oscillation converters are commonly used in high efficiency power supplies. This type of converter generally comprises an input rectifier and filter, high frequency inverter, control circuit and an output section. Typically, high frequency MOSFET switches are operated by varying duty cycle or frequency to maintain the output voltage at a desired level. The efficiency of such converters is limited by losses in the MOSFETs during turn-on and turn-off, particularly in pulse-width modulated (PWM) converters. Since the primary current of a power transformer is periodically interrupted by high voltage spikes, EMI is produced. In the case of balanced converters, it is difficult to achieve synchronized opening and closing of the MOSFET switches due to stray capacitances, inductance and noise, complex control circuity for providing properly formed and timed DC pulses is generally required. These conditions require the addition of numerous interference suppressor and protection circuits and result in increased converter size and complexity. Methods for reducing EMI for forced oscillation power converters include the use of snubbers, input filters as well as adoption of special control strategies. However, these methods further complicate the design process and appreciably increase production cost.
Self oscillating converters do not utilize as many components as forced oscillation converters and may not generate such high levels of EMI, however they suffer from switching speed limitations and power inefficiencies. Transformer coupled self oscillating converters are designed to trigger switching transistor turn-on and turn-off using either the saturation of the switching devices, saturation of the power transformer core, or saturation of an intermediate drive transformer. The technique of saturation switching transistors is limited to slower switching speeds and the energy required to fully saturate transformers causes power losses and results in compromised efficiencies.
Specifically, a longstanding type of self oscillating DC to DC converter is disclosed in U.S. Pat. No. 5,303,137 to Peterson. Peterson utilizes a MOSFET half-bridge configuration in which each transistor is alternately saturated. Once a transistor is saturated, current continues to flow in the transformer winding due to the magnetizing inductance of transformer and the reflected load current. This current discharges the voltage across a circuit capacitor to reduce the voltage across the primary winding, which in turn is coupled to the gate windings. As the current rises, the transistor comes out of saturation and turns itself on again causing a voltage drop to reappear across the transformer. However, since this switching technique relies on the transistor beta factor it cannot be implemented using MOSFETs and higher switching frequencies cannot be achieved.
Another type of self oscillating circuit drives transistor switches using the properties of core saturation, as described in U.S. Pat. No. 4,319,315 to Keeney, Jr. et al., where a DC to DC converter uses a saturable transformer having a center-tap and resistive network interposed among four sequentially operating transistors. The transistors cause one side of the input DC voltage to be sequentially applied to opposite sides of a primary winding of the transformer which, in turn, cause the transformer to be excited into positive and then negative saturation conditions. When the transformer core saturates, appropriate gate drive voltages collapse and the associated transistors switches are turned off. While core saturating transformer converters are simpler than forced oscillating converters, since the core must be fluxed from one end to another in order to fully saturate, significant power losses result and overall efficiency is reduced.
Finally, another type of self oscillating converter utilizes resonance effects within an oscillating circuit, such as the converter disclosed in U.S. Pat. No. 5,430,632 to Meszenyi where a pair of MOSFET transistors are configured in a half-bridge configuration and coupled to a reactive network. The frequency of oscillation is determined by the gate-to-source capacitance of the transistors and the inductance of the drive transformer. This resonance converter suffers from increased complexity, sensitivity to parasitics and emissions due to its high operating frequency.
Thus, there is a need for a self oscillating power converter which achieves conventional power efficiencies using a minimal number of parts, which generates a significantly reduced amount of EMI, which provides increased reliability, and which can be operated at slower switching speeds to further reduce EMI emissions.